Rechargeable electrochemical cells that use nickel hydroxide positive electrodes and a metal hydride negative electrode are well known in the art. In fact, over the past several years metal hydride cells have gained wide spread market acceptance due to the fact that they incorporate desirable performance characteristics into a low cost relatively environmentally benign package. Examples of these desirable performance characteristics include high charge acceptance, relatively long cycle life and operation over a wide range of temperatures. Each of these performance characteristics represents improvements over the nickel cadmium and other battery systems known in the prior art.
Typically, metal hydride hydrogen storage electrodes are used as the negative electrode in the hydrogen storage system. The negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion. The reaction which takes place at the metal hydride electrode may be described by the following formula: ##STR1##
The reaction that takes place at the positive electrode of a nickel metal hydride cell is also a reversible reaction. In the case of the nickel hydroxide electrode, the positive electrode reaction is as follows: EQU Ni(OH).sub.2 +OH.sup.- .revreaction.NiOOH+H.sub.2 O+e.sup.-
The negative electrode of most metal hydride electrochemical cells can be characterized by one of two chemical formulas. The first is AB.sub.2 which describes a TiNi type battery systems such as described in, for example, U.S. Pat. No. 5,277,999. The second formula is AB.sub.5 which describes a LiNi.sub.5 type systems as described in, for example, U.S. Pat. No. 4,487,817. Substantially all metal hydride electrochemical cells fall into one of these two categories. Cadmium can also be considered a hydrogen storing material in the sense that it can store hydrogen as a hydroxide, i.e., Cd(OH).sub.2.
Regardless of the type of material, it is commonly the case that the electrolyte used between the two electrodes, i.e., the component of the battery which promotes ion transport between the two electrodes, is a liquid electrolyte and typically is a base such as KOH. Liquid electrolytes, while demonstrating acceptable ionic conductivity, tend to leak out of the cells into which they are sealed. While better manufacturing techniques have lessened the occurrence of leakage, cells do leak liquid electrolytes from time to time. Moreover, any electrolyte leakage in the cells lessens the amount of electrolyte available to the cell, thus reducing its effectiveness. Seals for wet electrochemical cells has been widely investigated for a number of years in order to try and develop systems which eliminate the liquid electrolyte leakage problem.
Electrolytes have been studied extensively during the past several years in an effort to develop solid state or substantially solid electrolytes which provide high ionic conductivity as required by virtually all electrochemical cells, in a dry or substantially dry environment so as to eliminate the problems of electrolyte leakage. Heretofore, acceptable solutions to this issue have not been developed.
Accordingly, there exists a need for a new type of electrolyte system for use in nickel metal hydride electrochemical cells which combines the mechanical stability and freedom from leakage offered by solid electrolytes, with the high ionic conductivity characteristic of liquid electrolytes.